Production of metal-organic frameworks

ABSTRACT

An apparatus for producing metal organic frameworks, comprising: a tubular flow reactor comprising a tubular body into which, in use, precursor compounds which form the metal organic framework are fed and flow, said tubular body including at least one annular loop.

This application is a Continuation of U.S. patent application Ser. No.15/172,942, filed 3 Jun. 2016, which is a Continuation of U.S. patentapplication Ser. No. 14/545,594, filed 27 May 2015, now U.S. Pat. No.9,630,163, and a Continuation of International Patent Application No.PCT/AU2015/000317, filed 27 May 2015 and which applications areincorporated herein by reference. To the extent appropriate, a claim ofpriority is made to each of the above disclosed applications.

TECHNICAL FIELD

The present invention generally relates to an apparatus, process andsystem for the production of metal-organic frameworks. The invention isparticularly applicable for production of metal-organic frameworks(MOFs) and it will be convenient to hereinafter disclose the inventionin relation to those exemplary applications.

BACKGROUND OF THE INVENTION

The following discussion of the background to the invention is intendedto facilitate an understanding of the invention. However, it should beappreciated that the discussion is not an acknowledgement or admissionthat any of the material referred to was published, known or part of thecommon general knowledge as at the priority date of the application.

Metal-Organic Frameworks (MOFs) are a class of promising porousmaterials having tunable functionality, large pore sizes and the highestknown surface areas. These characteristics are of high interest for amyriad of industrial applications such as gas storage, gas separation,drug delivery and catalysis. However, to date the cost of thesematerials has remained prohibitively high, thereby restricting theability of these materials to make a significant impact on prospectivemarkets or technologies. Very few MOFs described in academic literatureare commercially available, with that availability limited to smallquantities (grams).

An important requirement for accessing the potential applications ofMOFs is the ability to routinely synthesise MOF materials in largequantities (kg scale or higher) at an economic price point. Such aprocess needs to be a versatile, efficient and scalable synthesis thatis able to produce MOFs in large quantities in order to introduce thesematerials to real world applications.

However, traditional laboratory routes such as the classicalsolvothermal synthesis are difficult to scale up due to the extendedreaction times (˜24 hours) and low quality material yield. Furthermore,a wide variety of available synthetic synthesis methods have a singularnature providing an inherent inflexibility for any prospectiveproduction process.

One of the barriers to scaled-up MOF synthesis is that commonly MOFsnucleate at a reaction surface, meaning that the size of the reactionvessel becomes a significant parameter in the synthesis conditions.Consequently, reactions that proceed in small lab scale conditions arenot always successful when scaled up into larger vessels, limitingscaled up MOF chemistry to a small number of MOFs that are robust intheir preparation, each requiring bespoke equipment. Therefore a methodto conveniently expand the scale of production, keep sufficientresidence times, while minimising vessel geometry is extremelyattractive to applied MOF chemistry, offering a versatile route toproduction.

Continuous flow chemistry is renowned as a paradigm shifting approach tochemical synthesis. The improved heat and mass transfer available oftenleads to improved reaction yields, reduced reaction times, fasterreaction syntheses, new synthetic pathways, and broader green chemistryimplications.

Recent studies have reported that it is possible for MOFs to be producedby continuous processes. Gimeno-Fabra M. et al. Instant MOFs: continuoussynthesis of metal-organic frameworks by rapid solvent mixing. Chem.Commun. 48, 10642-10644 (2012) showed that use of a bespoketube-in-tube, counter-current mixing reactor at the high temperature of300° C. can lead to MOFs. It was also shown that small amounts of MOFs,within oil droplets, can be made in microfluidic reactors (see FaustiniM. et al. Microfluidic Approach toward Continuous and Ultra-FastSyn-thesis of Metal-Organic Framework Crystals and Hetero-Structures inConfined Microdroplets. J. Am. Chem. Soc. 135, 14619-14626 (2013) andPaseta L. et al. Accelerating the controlled synthesis of MOFs by amicrofluidic approach: a nanoliter continuous reactor. ACS Appl. Mater.Interfaces 5, 9405-9410 (2013)). In 2013 Kim K.-J. et al. (High-ratesynthesis of Cu-BTC metal-organic frameworks. Chem. Commun. 49,11518-11520 (2013)) reported a proof of concept mesoscale flowproduction of HKUST-1 using a continuos flow reactor comprising a 30 cmlong and 1.59 mm I.D. stainless steel pipe. It is noted that the MOFsproduced had moderate surface area at low scale. All of these earlyreports are promising steps towards production of MOFs at scale.However, in order for this to be viable, pure MOFs must be readilyattainable without a loss in product quality.

Given the wide array of MOFs known, and the likelihood of a large rangeof applications each requiring different MOFs in the future, a versatileproduction technique is crucial. It would therefore be desirable toprovide a new and/or improved method and apparatus for producing MOFs.

SUMMARY OF THE INVENTION

The present invention provides a new and/or improved apparatus, systemand process for producing a metal organic framework.

A first aspect of the present invention provides an apparatus forproducing metal organic frameworks, comprising:

a tubular flow reactor comprising a tubular body into which, in use,precursor compounds which form the metal organic framework are fed andflow, said tubular body including at least one annular loop.

A second aspect of the present invention provides use of a tubular flowreactor for producing metal organic frameworks, wherein the tubular flowreactor comprises a tubular body including at least one annular loop,and in use precursor compounds for forming the metal organic frameworkare fed and flow through the tubular body.

The present invention provides a continuous flow chemistry process,system and apparatus for the production of MOFs which is applicable to alarge number of MOFs with different reaction conditions. Continuous flowproduction of MOF materials allows MOFs to be continuously produced forextended periods of time. Furthermore, a continuous flow approach canprovide a reaction rate that is higher, typically significantly higherthan any previously reported values, and is capable of producing athigher space time yields than other commercial manufacturing processes.The process of the present invention is further scalable without alosses in yield or surface area of the material with concomitant controlover particle size. The apparatus and process of the present inventiontherefore demonstrate production quantities approaching those requiredfor broad application.

It should be understood that flow reactors may also be referred toherein as continuous flow reactors. Furthermore, it should beappreciated that the features discussed below in connection with thefirst aspect of the present invention (a tubular flow reactor) equallyapply to the second aspect of the present invention (use of a tubularflow reactor).

The advantage of the present process and apparatus over prior processesis in at least part a result of the configuration of the tubular flowreactor. The tubular flow reactor of the present invention comprises atleast one annular loop, and preferably a plurality of annular loops. Inexemplary embodiments, the tubular flow reactor comprises a coil orcoiled reactor. A coil reactor advantageously allows precise andhomogeneous control of the temperature and mixing of the reagents,reducing the reaction time, achieving highest material quality, highestyields and control over the particle size. The coil tubular body of thetubular flow reactor of the present invention therefore enables morehomogeneous heating and better mixing and as consequence higher qualitymaterials and less reaction time in comparison to prior publishedstudies of MOFs produced by continuous processes. Other reactors may beemployed to suit the design of ancillary equipment or varied processconditions. For example where prolonged contact with the energy transferdevice is less critical a tube-in-shell reactor arrangement may beemployed to give higher throughput. However an annular loopconfiguration is preferred as it allows efficient energy transfer to thereaction mixture using a very simple design that has a low cost tomanufacture.

The at least one conduit of the tubular reactor of the present inventionincludes a device, element or arrangement which supplies energy to thereaction mixture. This energy can be, but is not limited to heat,electromagnetic energy, sonic energy. The tubular reactor itself ispreferably designed such that the transfer of energy to the reactionmixture is as efficient as possible and may therefore be in the form ofa single tube, tube and shell, plate and frame, pillow panel orcomplex-structured reactor type.

The annular loops of the tubular body can be arranged in any suitableconfiguration. The annular loops of the tubular body may be curvedthrough 0 to 360 degrees of curvature in any direction and any curvesmay be reversed or orthogonal to previous or following curved sectionsof the tubular body. The annular loops may follow an open loop(including straight), serpentine or annular/helical configurations. Thediameter of the tubular body may vary along its length and structures orsurface treatments included inside the tubular body to alter the flowpath of the materials passing through it. The tubular body may bepermeable along its length to allow the introduction or withdrawal offluids to or from the tubular body. In some embodiments, each annularloop is radially centred about and axially spaced along a central axis.The annular loops can therefore form a substantially tubular shaped coilradially centred about the central axis. Again, in exemplary embodimentsthe annular loops comprise a coil, preferably a helical coil. In someembodiments, the tubular flow reactor comprises a capillary tubular flowreactor. However, it should be appreciated that not all embodiments arenecessarily capillary tubular flow reactors. It should be understoodthat the internal diameter of the tubular body of the tubular flowreactor can be sized for various applications. In some embodiments, theinternal diameter of the tubular body is between 0.5 mm and 50 mm,preferably between 1 and 25 mm, more preferably from 1 to 15 mm.

The dimensions and configuration of each annular loop can vary dependingon the application and scale of production. In embodiments, the averageradius of each annular loop is between 10 and 1000 mm. In otherembodiments, the average radius of each annular loop is between 20 and500 mm, preferably between 40 and 200 mm. Similarly, in some embodimentsthe length of the coil is greater than 50 mm, preferably greater than100 mm, more preferably between 20 and 200 mm. In some embodiments, thelength of the coil is between 200 and 1000 mm. It should be noted thatthe uncoiled length of the tube would be significantly longer, in somecases being in excess of 10 m, in some cases in excess of 20 m.

The tubular body of the tubular flow reactor can comprise one or morelength of coil. In some embodiments, the tubular body comprises a singlelength of coil. In other embodiments, the tubular body comprises atleast two fluidly connected coils. It should be understood that thosefluidly connected coils could be connected in series and/or parallelwithin the overall tubular flow reactor. In some embodiments, thefluidly connected coils are connected in series to increase the reactorlength of the tubular body. In some embodiments, the fluidly connectedcoils are connected in parallel in order to increase the flow capacityof the tubular body. A combination of parallel and series connectedcoils can also be used. It is noted that a parallel, multiple coilarrangement would enable a multiple component MOF to be thermallytreated in stages and then do a final pass through the same heatedvessel.

It should be appreciated that flow reactors can readily be operated withmultiple flow lines making the scale up to large production quantitiesrelatively straight forward. In particular, it can be more effective andefficient to “number-up” (i.e. scale up through repetition orreplication) flow lines to produce a given quantity of MOF. For example,a flow reactor for producing 0.2 g/unit time of MOF material can bereadily be “numbered up” to produce, 2 g, 20 g, 200 g or 2 kg/unit timeetc. of MOF material. In one embodiment, the flow reactor is a tubularcoil flow reactor in which the tubular body is constructed ofperfluoroalkoxy alkane (PFA) or metal, for example stainless steel.However, it should be appreciated that the tubular body could beconstructed of any suitable material including various plastics, metals,ceramics or the like. In this respect, the materials of construction(and wall thicknesses) are preferably selected to deal with thetemperature and pressure required in the reactor, and are chemicallycompatible with the reagents, MOF product and byproducts. It should beappreciated that the internal surface of the tubular body can be coatedto activate reactions, or repress side reactions, or for other purposes.

It should be appreciated that the use of annular loops, preferably coilsin the tubular reactor can allow very high surface are for a smallfootprint. High surface area increases the amount of MOF that can beproduced by the apparatus, in some cases allowing many kgs of MOF to beproduced using the apparatus of the present invention. In some cases,coiling may also assist in the prevention of clogging/blockage of thetubes of the tubular reactor via the velocity/annular velocity andcentrifugal force of the fluid generated therein.

It should be appreciated that the use of annular loops, preferably coilsin the tubular reactor can allow very high surface are for a smallfootprint. High surface area increases the amount of MOF that can beproduced by the apparatus, in some cases allowing many kgs of MOF to beproduced using the apparatus of the present invention. In some cases,coiling may also assist in the prevention of clogging/blockage of thetubes of the tubular reactor via the velocity/annular velocity andcentrifugal force of the fluid generated therein.

The tubular body is preferably heated. The tubular body can be heated byany suitable arrangement. In some embodiments, the tubular body iscovered by or otherwise in contact with a heating arrangement, forexample a heating element or the like. In some embodiments, a heatingfluid such as gas or liquid is ultilised. In other embodiments, thetubular body can be heated by a number of means including gas (such asair, post combustion gases, steam), liquid (water, heating fluid such assilicone oil), or electrically. In some embodiments, the tubular body islocated inside a heated housing. The precursor compounds flowing throughthe tubular body are heated to a suitable temperature conducive to MOFformation from these precursor compounds. The particular temperaturedepends on the reaction chemistry and desired reaction kinetics informing a particular MOF. However, in a number of embodiments thetubular body heats the precursor compounds to a temperature of between20 and 200° C., preferably between 25 and 150° C., more preferablybetween 25 and 130° C.

In some embodiments, the energy source for the synthesis of MOFs in thetubular flow reactor is photochemical in nature. In other embodimentsthe energy source is light based. In other embodiments, the energysource may result from ultrasonication, microwave heating, cooling, orthe like.

The preferred pressure in the reactor is between 0 and 30 bar,preferably between 5 and 10 bars. However, it should be appreciated thatpressure is a function of temperature of the fluid and therefore mayvary accordingly.

It should be appreciated that the tubular flow reactor of the presentinvention can include any number of additional features including (butnot limited to) in-line monitoring of reaction conditions, optical,thermal, pH probes, conductivity probes/sensors, particle sizedistribution (PSD), UV, IR, Laser Induced Breakdown Spectroscopy (LIBS)and the like.

A third aspect of the present invention provides a process for producingmetal organic frameworks which comprises:

introducing into an apparatus according to the first aspect of thepresent invention a solution comprising precursor compounds for formingthe metal organic framework in solvent; and

promoting a reaction within the tubular flow reactor to form the metalorganic framework.

It should be appreciated that in embodiments, the apparatus cancontinuously run to produce at least 1 kg/hr, preferably 2 kg/hr.However, it should be appreciated that the production rate will vary foreach different MOF because each MOF has different molecular weight anddifferent reactant and product concentrations. Furthermore, the yield ofMOF from the apparatus is preferably greater than 60%, more preferablygreater than 80%, and in some embodiments greater than 95%. For example,for in embodiments, the maximum yield is 100% (for the AluminiumFumarate) using in both cases the maximum concentration of precursorsbased on their solubility.

The precursor compounds can be introduced into the apparatus in avariety of different regimes. In some embodiments, the precursorcompounds are provided in at least two different precursor solutionscontaining different precursor compounds, the precursor solution beingmixed prior to introduction into the tubular body. In these embodiments,the precursor solutions are preferably mixed within a mixing vesselprior to introduction into the tubular body. It should be appreciatedthat precursors can either be dissolved in the solution or be providedas an undissolved component/solid for example a dispersion.

In other embodiments, the precursor solutions are mixed through inlinemixing in a feed conduit fluidly connected to an inlet of the tubularbody. A number of inline mixing arrangements can be used. For example,inline mixing can comprises one or more mixing joints, preferably T-Y-or cross junctions, annular feed systems or a plethora of channelsbetween feed flows of the two or more precursor solutions, such thatmixing of the components occurs in an optimal manner. In someembodiments, the mixing arrangement comprises an mixing element which isinsertable into one or more of the conduits. In some embodiments, theprecursor solutions are mixed using static mixers. Static mixer may beeasier to use for reaction tubes of larger internal diameter. A staticmixer or mixer present in a conduit may not be heated directly, butrather receive heat, via conduction (i.e. in direct contact with theconduit) or via convection. The static mixers can be incorporated intothe tube before entry into the reactor, or in the tube which is in theheated zone, or both. As such, the static mixer section can be heated inany of these placements. Heating to the static mixer would be viaconduction (if the static mixer is in direct/intimate contact with theinternal wall of the tube, or via convection. Differentshapes/geometries of static mixers are possible and would be a functionof the degree of mixing/flow patterns required, and possibly tofacilitate/tune the balance between nucleation vs particle growth.

In yet other embodiments, the precursor compounds are provided in atleast two different precursor solutions containing different precursorcompounds, the precursor solutions being mixed after introduction intothe tubular body. It should be appreciated that the different precursorsolutions can be fed into the same inlet or separate inlets. However,where the different precursor solutions are fed into the same inlet, itis preferable that the two or more precursor solutions are mixed at orproximate that inlet.

The apparatus of the present invention preferably includes a flowrestriction device downstream of the tubular reactor to control/set thedesired pressure required in the reactor. Preferably, the flowrestriction device maintains a constant the pressure of the flow streamin the reactor. The flow restriction device may be in the form of aback-pressure controller of fixed spring loading, manually set or ofautomated design. Alternatively the flow restriction device may be asimple valve operated manually or via an automated control system, or afixed orifice. In some embodiments, the flow restriction devicecomprises a diaphragm sensing back pressure regulator from Swagelok(Series KBP). Control of the flow restrictor may be linked to operationof the tubular reactor by feedback loop for example using pressure ortemperature sensors and the degree of flow restriction varied to controlthe pressure or pressure profile achieved in the reactor duringoperation. The back pressure regulator is located after the reactor andused to prevent the reaction product, the MOFs, from blockage up thereaction tube and preventing continuous flow It has been surprising tofind that particulate compounds, such as MOFs, can be made in such smalldiameter tubing without blockage and that the use of annular loops andback pressure regulation has allowed continuos production of kilogramsof material over very short periods of time.

In some embodiments of the present invention, the process furthercomprises the step of:

separating the MOF from the MOF containing solution.

This separation can be achieved using a number of unit processes,including centrifuging, filtration or the like. However, in someembodiments, this separation is achieved using the step of:

applying a high frequency ultrasound of at least 20 kHz, preferablybetween 20 to 4 MHz, more preferably 500 kHz to 2 MHz, yet morepreferably between 800 kHz and 2 MHz, and yet more preferably between 1MHz and 2 MHz to the MOF containing solution to a MOF containingsolution sourced from the tubular flow reactor, thereby substantiallyseparating the MOF material from solution as an aggregated sedimentwhich settles out of the MOF containing solution.

In some embodiments, the apparatus of the first and second aspects ofthe invention can similarly further include an ultrasonic and/ormegasonic separation apparatus. Thus, in some embodiments the system andapparatus further includes an apparatus for separating a metal organicframework (MOF) from a solution, comprising:

a housing having a reservoir capable of receiving a MOF containingsolution; and

a high frequency ultrasound transducer operatively connected to thereservoir and capable of applying frequencies of at least 20 kHz,preferably between 20 to 4 MHz, more preferably 500 kHz to 2 MHz, yetmore preferably between 800 kHz and 2 MHz, and yet more preferablybetween 1 MHz and 2 MHz to the MOF containing solution to the MOFcontaining solution.

The separation apparatus uses a high frequency ultrasound and is appliedto that MOF containing solution to effect separation of the MOF from thesolution. The apparatus can also be used for a washing or purificationmethod, in which the MOF includes at least one contaminant and theapparatus is used to separate those contaminants from the MOF insolution.

It should be appreciated that the separation apparatus could beintegrally incorporated into the structure of the tubular reactor toform a single apparatus or arrangement. Alternatively, the separationapparatus could be connected, preferably fluidly connected to thetubular reactor, and thus provided a further unit/process step in theoverall MOF production process.

It should be appreciated that separation in this washing and purifyingcontext broadly encompasses a number of unit processes including washingprocesses, purification processes, polishing processes and the like. Allof these processes involve the separation of a product (in the presentinvention a MOF) from a contaminant or other material. It should beappreciated that all these process functions and similar processes areincorporated into the scope of the present invention.

Ultrasonic separation involves the application of high frequencyultrasound or megasonic frequencies of >20 kHz to a MOF containingsolution. Acoustic radiation from the applied frequencies aggregate MOFstowards pressure nodes formed within the MOF-containing solution. Theaggregated MOF material tends to sediment out of solution at a greatlyaccelerated rate to the bottom of a container or separation chamberhousing the MOF containing solution. Ultrasonic and/or megasonicoperation involves no moving parts, has a low surface area of contactwith the fluid (i.e. lower capacity for fouling, ease of cleaning) andallows continuous separation, washing and/or purification of MOFs.Furthermore, the simplicity and speed of the process enables the processto be scaled, and applied economically to an existing MOF productionmethod.

In many embodiments, the apparatus for separating a metal organicframework (MOF) further includes an acoustic reflector surface spacedapart from the transducer within the housing, the transducer, in use,being operated to reflect said applied high frequency ultrasound off theacoustic reflector surface. The transducer is therefore operated toapply a high frequency ultrasound to the MOF containing solution and toreflect said applied ultrasound from the acoustic reflector surface. Theuse of an acoustic reflector surface assists in the formation of astanding wave field required to form pressure nodes where particles arecollected for cleaning or separation. This substantially separates theMOF material from solution as a aggregated sediment which settles out ofthe MOF containing solution.

The acoustic reflector surface is generally located in front of thetransducer, and spaced apart from that transducer. In some embodiments,the transducer is located proximate or at one wall or side of thehousing, and the acoustic reflector surface is located proximate or atan opposite wall or side of the housing.

The frequency of the applied high frequency ultrasound is important inthe function and effect of the separation. Whilst the preferredfrequency depends on factors such as MOF composition, particle size,solution composition and the like, the general ranges of applied highfrequency ultrasound are as follows: In some embodiments, the appliedhigh frequency ultrasound is between 20 to 4 MHz, preferably 500 kHz to2 MHz, more preferably between 800 kHz and 2 MHz, and yet morepreferably between 1 MHz and 2 MHz. In some embodiments, the appliedhigh frequency ultrasound is greater than 1 MHz, preferably between 1MHz and 10 MHz, and more preferably between 1 and 4 MHz.

In some cases, it can be advantageous to move the applied high frequencyultrasound between a high frequency and a low frequency. In someembodiments, the applied high frequency ultrasound is cycled between ahigh frequency and a low frequency. Again, the selected frequenciesdepend on a number of factors. However, in some embodiments the highfrequency is between 400 kHz to 2 MHz and the low frequency is between20 kHz to 400 kHz. However, other embodiments the low frequency isbetween 20 kHz to 500 kHz and the high frequency is between 500 kHz to 2MHz.

The energy density of the applied high frequency ultrasound is anotherfactor which can effect separation. In some embodiments, the energydensity of the applied high frequency ultrasound is at least 25 kJ/kg,preferably between 100 kJ/kg to 250 kJ/kg.

In some embodiments, the process and apparatus of the present inventionhas the ability to achieve specificity of separation based on particlesize by tuning of the operation parameters such as frequency and energydensity. Thus, in some embodiments at least one of frequency or energydensity of the applied high frequency ultrasound is tuned to selectivelyseparate MOF and contaminants based on a specific particle size.

MOF material is extremely porous and therefore contaminant species in asolution can be trapped or otherwise located in the pores of the MOFmaterial. The process of the present invention can be used forseparation and/or purification of MOFs from such contaminants, and moreparticularly contaminants in the pores of a MOF. Thus, in someembodiments, the metal organic framework (MOF) includes at least onecontaminant, and the method substantially separates the contaminant fromthe MOF within the solution. The contaminant is preferably left insolution and the MOF settles at or proximate to the bottom of thesolution. Again, this separation includes contaminants in the pores ofthe MOF.

The Applicant considers that the size, material and/or geometry of thevessel or housing used for ultrasonic and/or megasonicsseparation/activation may have an effect on the outcome (degree,efficiency or the like) of ultrasonic and/or megasonic separationprocess of MOFs. Similarly, the positioning, arrangement and alignmentof transducers within a separation apparatus may have an effect on theoutcome (degree, efficiency or the like) of ultrasonic and/or megasonicseparation process of MOF.

The transducer can be positioned in any suitable location in relation tothe housing to apply the ultrasonic and/or megasonic frequencies to theMOF containing liquid received within the reservoir. In someembodiments, the housing comprises a container including at least onewall position to contact the MOF containing solution, and the transduceris high frequency ultrasound transducer which is positioned within thereservoir or in engagement with the at least one wall. In each case, thetransducer is operable to apply ultrasonic and/or megasonic frequenciesto a MOF containing solution housed in the reservoir.

The transducer can comprise any suitable high frequency ultrasoundtransducer. In some embodiments, the high frequency ultrasoundtransducer comprises a plate transducer.

The acoustic reflection of the applied frequencies can assist the MOFseparation process. Accordingly, in some embodiments the housingincludes at least one reflector surface designed to reflect the appliedfrequencies within the reservoir.

The MOF content is preferably separated from the solution followingsedimentation at the bottom of the solution. The process thereforepreferable further comprises the step of separating the MOF from thesolution. Separation can be achieved using any number of separationprocess steps including but not limited to decanting, filtration,evaporation, centrifugation, gravity separation, flotation, magneticseparation, spray drying or the like.

A fourth aspect of the present invention provides a system for producinga metal organic framework (MOF), comprising:

an apparatus for forming a metal organic framework from precursormaterials according to the first aspect of the present invention; and

an apparatus for washing and/or purifying the metal organic framework,comprising: a housing having a reservoir capable of receiving a MOFcontaining solution from the reactor; and a high frequency ultrasoundtransducer operatively connected to the reservoir and capable ofapplying frequencies of at least 20 kHz, preferably between 20 to 4 MHz,more preferably 500 kHz to 2 MHz, yet more preferably between 800 kHzand 2 MHz, and yet more preferably between 1 MHz and 2 MHz to the MOFcontaining solution to the MOF containing solution.

In many embodiments, the apparatus for washing and/or purifying themetal organic framework further includes an acoustic reflector surfacespaced apart from the transducer within the housing, the transducer, inuse, being operated to reflect said applied high frequency ultrasoundoff the acoustic reflector surface.

A large variety of MOFs or MOF materials can be produced using theapparatus, process and system of the present invention.

It should be appreciated that Metal Organic Frameworks (MOFs) (alsoknown as coordination polymers) or MOFs are a class of hybrid crystalmaterials where metal ions or small inorganic nano-clusters are linkedinto one-, two- or three-dimensional networks by multi-functionalorganic linkers. In this sense, MOF is a coordination network withorganic ligands containing potential voids. A coordination network is acoordination compound extending, through repeating coordinationentities, in one dimension, but with cross-links between two or moreindividual chains, loops, or spiro-links, or a coordination compoundextending through repeating coordination entities in two or threedimensions and finally a coordination polymer is a coordination compoundwith repeating coordination entities extending in one, two, or threedimensions.

MOFs have many appealing features having surface areas of thousands ofsquare meters per gram, extremely low density, interconnected cavitiesand very narrow porosity distributions. A variety of open micro- andmesoporous structures can be developed, leading to materials withextreme surface area.

Examples of metal organic frameworks which may be suitable for use inthe present invention include those commonly known in the art asMOF-177, MOF-5, IRMOF-1, IRMOF-8, Al-fum (Aluminium fumarate) and MIL-53(aluminium terephthalate) Zr-Fum (Zirconium fumarate), UiO-66, HKUST-1,NOTT-400, MOF-. It should be appreciated that the present invention issuitable for use with a large number of MOFs and should therefore not belimited to the exemplified MOF structures in the present application.

MOFs used in the process of the present invention preferably comprise aplurality of metal clusters, each metal cluster including one or moremetal ions; and a plurality of charged multidentate linking ligandsconnecting adjacent metal clusters. Such MOFs can therefore be moregenerally defined by the charged multidentate linking ligands connectingadjacent metal clusters which are used to form each MOF. The MOFprecursors can include one or more of the metal cluster or a metallicsalt thereof and/or the multidentate linking ligands which form thefinal MOF.

Each metal cluster preferably includes one or more metal ions. As usedherein, the term “cluster” means a moiety containing one or more atomsor ions of one or more metals or metalloids. This definition embracessingle atoms or ions and groups of atoms or ions that optionally includeligands or covalently bonded groups. Each cluster preferably comprisestwo or more metal or metalloid ions (hereinafter jointly referred to as“metal ions”) and each ligand of the plurality of multidentate ligandincludes two or more carboxylates.

In some embodiments, at least one ligand of the plurality ofmultidentate ligand comprises an organic ligand which is at leastbidentate and is selected from the group consisting of formic acid,acetic acid, oxalic acid, propanoic acid, butanedioic acid,(E)-butenedioic acid, benzene-1,4-dicarboxylic acid,benzene-1,3-dicarboxylic acid, benzene-1,3,5-tricarboxylic acid,2-amino-1,4-benzenedicarboxylic acid, 2-bromo-1,4-benzenedicarboxylicacid, biphenyl-4,4′-dicarboxylic acid,biphenyl-3,3′,5,5′-tetracarboxylic acid, biphenyl-3,4′,5-tricarboxylicacid, 2,5-dihydroxy-1,4-benzenedicarboxylic acid,1,3,5-tris(4-carboxyphenyl)benzene, (2E,4E)-hexa-2,4-dienedioic acid,1,4-naphthalenedicarboxylic acid, pyrene-2,7-dicarboxylic acid,4,5,9,10-tetrahydropyrene-2,7-dicarboxylic acid, aspartic acid, glutamicacid, adenine, 4,4′-bypiridine, pyrimidine, pyrazine,pyridine-4-carboxylic acid, pyridine-3-carboxylic acid, imidazole,1H-benzimidazole, 2-methyl-1H-imidazole, and mixtures thereof.

Typically, the metal ion is selected from the group consisting of Group1 through 16 metals of the IUPAC Periodic Table of the Elementsincluding actinides, and lanthanides, and combinations thereof.Preferably, the metal ion is selected from the group consisting of Li⁺,Na⁺, K⁺, Rb⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, Zr⁴⁺,Hf⁴⁺, V⁵⁺, V⁴⁺, V³⁺, V²⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, W³⁺, Mn³⁺, Mn²⁺, Re³⁺,Re²⁺, Fe³⁺, Fe²⁺, Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Co²⁺, Rh²⁺, Rh⁺, Ir²⁺,Ir⁺, Ni²⁺, Ni⁺, Pd²⁺, Pd+, Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺,Hg²⁺, B³⁺, B⁵⁺, Al³⁺, Ga³⁺, In³⁺, Tl³⁺, Si⁴⁺, Si²⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺,Sn²⁺, Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺, As⁺, Sb⁵⁺, Sb³⁺, Sb⁺, Bi⁵⁺, Bi³⁺, Bi⁺ andcombinations thereof.

Typically, the cluster has formula M_(m)X_(n) where M is the metal ion,X is selected from the group consisting of Group 13 through Group 17anion, m is an number from 1 to 10, and n is a number selected to chargebalance the cluster so that the cluster has a predetermined electriccharge

Preferably, X is selected from the group consisting of O²⁻, N³⁻ and S²⁻.Preferably, M is selected from the group consisting of Li⁺, K⁺, Na⁺,Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, V²⁺, V³⁺, V⁴⁺, V⁵⁺, Mn²⁺, Re²⁺, Fe²⁺, Fe³,Ru³′, Ru²⁺, Os²⁺, Co²⁺, Rh²⁺, Ir²⁺, Ni²⁺, Pd²⁺, Pt²⁺, Cu²⁺, Zn²⁺, Cd²⁺,Hg²⁺, Si²⁺, Ge²⁺, Sn²⁺, and Pb²⁺. More preferably M is Zn²⁺ and X isO²⁻.

Typically, the multidentate linking ligand has 6 or more atoms that areincorporated in aromatic rings or non-aromatic rings. Preferably, themultidentate linking ligand has 12 or more atoms that are incorporatedin aromatic rings or non-aromatic rings. More preferably, the one ormore multidentate linking ligands comprise a ligand selected from thegroup consisting of ligands having formulae 1 through 27:

wherein X is hydrogen, —NHR, —N(R)₂, halides, C₁₋₁₀ alkyl, C₆₋₁₈ aryl,or C₆₋₁₈ aralkyl, —NH₂, alkenyl, alkynyl, —Oalkyl, —NH(aryl),cycloalkyl, cycloalkenyl, cycloalkynyl, —(CO)R, —(SO₂)R, —(CO₂)R—SH,—S(alkyl), —SO₃H, —SO³⁻M⁺, —COOH, —COO-M⁺, —PO₃H₂—, —PO₃H-M⁺, —PO₃²⁻M²⁺, or —PO₃ ²⁻M²⁺, —NO₂, —CO₂H, silyl derivatives; boranederivatives; and ferrocenes and other metallocenes; M is a metal atom,and R is C₁₋₁₀ alkyl.

In one embodiment, the multidentate linking ligand comprises a ligandhaving formula 3 previously described. In another embodiment, themultidentate linking ligand comprises a ligand having formula 18(“BTB”). In a further embodiment, the multidentate linking ligandcomprises a ligand having formula 14.

BRIEF DESCRIPTION OF THE DRAWINGS

The application file contains at least one drawing executed in color.Copies of this patent application publication with color drawings willbe provided by the Office upon request and payment of the necessary fee.The present invention will now be described with reference to thefigures of the accompanying drawings, which illustrate particularpreferred embodiments of the present invention, wherein:

FIG. 1A provides a schematic representation showing the general flowreactor setup for the production of metal-organic framework solutionsaccording to one embodiment of the present invention.

FIG. 1B provides a schematic representation showing the general flowreactor setup for the production of metal-organic framework solutionsaccording to another embodiment of the present invention.

FIG. 2A provides a schematic representation showing the general flowreactor setup for the production of metal-organic framework solutionsaccording to yet another embodiment of the present invention.

FIG. 2B provides a schematic representation showing one embodiment ofthe coil flow reactor setup for the production of metal-organicframework solutions according to yet another embodiment of the presentinvention.

FIG. 2C provides a schematic representation showing another embodimentof the coil flow reactor setup for the production of metal-organicframework solutions according to yet another embodiment of the presentinvention.

FIG. 3 provides characterization data of a) HKUST-1, b) UiO-66 and c)NOTT-400 crystals obtained by flow chemistry using a total flow rate of2 mL·min⁻¹ respectively. Comparisons of the XRPD patterns obtained byflow (green) with simulated structures (black). SEM images of thecrystals obtained by flow chemistry.

FIG. 4 provides representative SEM images of the HKUST-1 crystalssynthesized by flow chemistry at 806° C. after 1, 2 and 10 minuteresidence times showing control over particle size (top). Scale bar: 500nm. Overview diagram of the influence of reaction parameters on productsynthesised based on data presented in the table (bottom). Productionquality is defined as the product of BET surface area and percentageyield. Data have been normalised such that the maximum value for eachparameter is set to unity.

FIG. 5 provides representations of BET surface area, SA_(BET), withrespect to the different concentrations of copper used for the synthesisof HKUST-1 at 80° C. (a) and at 140° C. (b), using different flow rates.

FIG. 6 provides SEM image and XRPD patterns of the HKUST-1 crystalssynthesized by flow chemistry at 140° C. and using a flow rate of 20mL·min⁻¹ (green), compared with the simulated XRPD pattern of HKUST-1(black). This XRPD pattern was collected using Cu Kα radiation.

FIG. 7 provides a) a photograph of the ultrasonic/megasonic separatorset-up with a high frequency system; b) a photograph of one 200 kHzplate transducer used in the reactor set up shown in (a); and c) aschematic of a standing wave pattern formed by the superimposition of areflected sound wave within the separator shown in (a).

FIG. 8 provides three photographs of solution being treated in aseparator shown in FIG. 7 at specific times (1 minute, 4 minutes and 10minutes) during a separation process according to one embodiment of thepresent invention.

FIG. 9 provides a photographic comparison and a comparison plot of thebackscattering and transmission data of the supernatant collected fromthe first separation of the MOF solution using centrifuge andultrasonic/megasonic separator for (A) Al-Fumarate supernatant; and (B)MIL-53 (Al) supernatant.

DETAILED DESCRIPTION

The present invention provides a new continuous flow chemistryapparatus, system and process for producing a large number of metalorganic frameworks even when requiring a number of different reactionconditions.

The apparatus of the present invention comprises a tubular flow reactorcomprising a tubular body into which, in use, precursor compounds whichform the metal organic framework are fed and flow, said tubular bodyincluding at least one annular loop. In exemplary embodiments, thetubular body comprises a coil. The tubular flow reactor thereforecomprises a coiled or coil tubular flow reactor.

The Inventors have found that the use of a coiled reactor enables morehomogeneous heating and better mixing and as consequence higher qualitymaterials and less reaction time in comparison to prior publishedstudies of MOFs produced by continuous processes. It is also thoughtthat coiling may assist in some embodiments in preventing clogging ofthe tubular reactor allowing for “continuous” use resulting in largescale production. The present invention therefore provides a fast,cost-effective environmentally friendly strategy to produce high-qualityMOF materials at a large scale.

Advantageously, the process is capable for being scaled (more than30-fold) without a loss in yield or surface area in the material with acontrol over particle size. The present invention can therefore permitlarge-scale production of MOFs at drastically reduced costs, allowingcommercialisation of these MOFs for many potential real worldapplications. The present invention provides a fast, cost-effectiveenvironmentally friendly strategy to produce high-quality MOF materialsat a large scale.

The flow reactor can have a large number of different process flowconfigurations:

FIG. 1A illustrates a first example flow diagram of a first continuousprocess 100 for producing MOF. In this process, a single feedstock tank101 is used to feed a continuous flow reactor 105. The continuous flowreactor 105 comprises a coil reactor comprising a plurality of annularloops or turns centered about a centreline. Feedstock tank 101 includesa solution of MOF precursor compounds mixed together in solvent. Thissolution is then pumped 103 into the continuous flow reactor 105 towhich heat is applied to induce reaction between the MOF precursorcompounds. The produced MOF is collected in the product tank 106.

FIG. 1B illustrates a second example flow diagram of a second continuousprocess 200 for producing MOF. In this process, a first feedstock tank201 includes a solution of a first precursor compound(s) in solvent. Asecond feedstock tank 202, includes a second precursor compound(s) insolvent. The solutions from each of the first and second feedstock tanks201 and 202 are then pumped 203 to a T-piece mixer 204 (which in otherembodiments could be Y- or cross junction mixer) where their flows arecombined and passed through to continuous flow reactor 205. Thecontinuous flow reactor 205 comprises a coil reactor comprising aplurality of annular loops or turns centered about a centreline. Heat isthen applied to the reactor 205 to which heat is applied to inducereaction between the MOF precursor compounds. The produced MOF iscollected in the product tank 206.

FIG. 2A illustrates a third example flow diagram of a third continuousprocess 300 for producing MOF. The flow diagram is similar to the flowdiagram shown in FIG. 1B with the exception that the flow reactor 305comprises two series connected coil reactors. Each continuous flowreactor 305 comprises a coil reactor comprising a plurality of annularloops or turns centered about a centreline. In this process, a firstfeedstock tank 301 includes a solution of a first precursor compound(s)in solvent. A second feedstock tank 302 includes a second precursorcompound(s) in solvent. The solutions from each feedstock tanks 301 and302 are then pumped 303 to the T-piece/mixer 304 where their flows arecombined and passed through to continuous flow reactor 305. Heat is thenapplied to the reactor 305 to which heat is applied to induce reactionbetween the MOF precursor compounds. Reactor 305 comprises two coiledreactors fluidly liked in series. This arrangement increases thereactive length of the overall flow reactor 305. It should beappreciated that any number of coiled tubular reactors could beconnected in series. The produced MOF is collected in the product tank306.

In each of the systems shown in FIGS. 1A, 1B and 2A, the reactionsolution is transferred via a flow line and introduced into the flowreactor 105, 205, 305. Introducing the reaction solution into the flowreactor 105, 205, 305 can be facilitated by any suitable means, but thiswill generally be by action of a pump 103, 203, 303. Those skilled inthe art will be able to select a suitable pump 103, 203, 303 for thepurpose of transferring the reaction solution from the vessel 101, 201,202, 301, 302 along the flow line and introducing it to the flow reactor105, 205, 305. The flow line is of a tubular type herein described andin effect forms the flow reactor 105, 205, 305 by being shaped into acoil configuration. The distinction between the flow line and the flowreactor 105, 205, 305 is that the flow reactor 105, 205, 305 is adesignated section of the flow line where formation of the MOF from theprecursor solutions is to be promoted. Promoting the formation of theMOF is shown by way of application of appropriate heat to the flowreactor 105, 205, 305. The coiled section of the flow line is thenreadily demarcated as the flow reactor 105, 205, 305.

It will be appreciated that the illustrated process can be operatedcontinuously by ensuring that vessel 101, 201, 202, 301, 302 ismaintained with reaction solution. Multiple flow lines can of coursealso be used to form the flow reactor 105, 205, 305 so as to increasethe volume of reaction solution drawn from vessel 101, 201, 202, 301,302 and thereby increase the volume of MOFs produced.

FIGS. 2B and 2C shows particular embodiments of flow reactor 405 whichcan be used in the flow set up shown in FIGS. 1A, 1B and 2A. Theembodiment of the flow reactor 405 shown in 2B comprises an elongatecoil 406 including aligned inlet and outlet 412 housed within a tubularhousing 410. The elongate coil 406 comprises a coil reactor comprising aplurality of annular loops or turns centered about a centreline(described in more detail below). The tubular housing is metallic,preferably stainless steel and includes two bulkhead ends 414, 415 whichare sealed via o-rings 417 to the main body of the housing using bolts419. In use, the MOF precursor fluid flows through the coil 406, whilstheating fluid passes through the housing 410. Temperature measurement ofthe contents on the shell side is via components 418. In this respect,the coil 406 and housing 410 is heated via heating inlet and outlet portconnections 421 and 422 through which heated fluid, for example aheating gas such as nitrogen or heating fluid such as an oil or thelike, are fed and extracted to heat the elongate coil 406. The coil 406can have any suitable dimensions. In one embodiment, the coil is a 845mm long coil of 0.25″ stainless steel tubing having 90 turns of coildiameter ˜7480 mm (outer diameter) with 3 mm spacing between eachannular loop or turn. In another embodiment, the coil 406 is a 863 mmlong coil of 0.5″ stainless steel tubing having 56 turns of coildiameter ˜130 mm with 3 mm spacing between each annular loop or turn.with coil diameter of 130 mm (outer diameter). It should be noted thatthe illustrated tubes are made from stainless steel. However, the choiceof material is dependent on the chemistry of the MOF reaction, i.e. themetal salt, ligand and solvent used. Accordingly, plastics or otheralloys may also be used. It is noted that insulation is included on theoutside of the tubular housing 410 to limit heat loss.

The embodiment of the flow reactor 405 shown in FIG. 2C comprises a verysimilar set up to that shown in FIG. 2B with the exception that theinlet and outlet 412 of the elongate coil are on opposite ends of thehousing 410. Due to these similarities, the same reference numerals havebeen used for this embodiment, and the above description in relation tothe embodiment illustrated in FIG. 2B equally applies to the embodimentshown in FIG. 2C.

The apparatus, process and system of the present invention can furtherinclude a ultrasonic/megasonic separation apparatus that can separate ametal-organic framework (MOF) content from a solution. This separationapparatus has been found to purify the MOF, removing contaminants fromthe pores of the MOF and also improve the surface area of the treatedMOF, producing a purified MOF having a higher surface area thancomparable commercially available samples.

The Inventors have found that the use of ultrasonic and megasonicfrequencies not only separates MOF material/particles from othercomponents in a mother solution, but also purifies the separated MOFmaterial. MOF material is extremely porous and therefore contaminantspecies in a solution can be trapped or otherwise located in thesepores. This separation apparatus has been found to substantially removecontaminants from the pores of MOF material treated with this separationmethod and apparatus. This produces a desirable substantially pure MOFmaterial which is highly saleable. The use of ultrasonic and megasonicfrequencies has also been found to improve the surface area of the finalproduct, acting as an alternate process to the time consuming and costlycalcinations traditionally used for surface area improvement. Theprocess can therefore assist in maintaining MOF product quality i.e.porosity, thermal and chemical stability.

Ultrasonic and/or megasonic separation according to the presentinvention applies >20 kHz, in some cases >400 kHz, preferably between 20to 4 MHz, preferably 500 kHz to 2 MHz, more preferably between 800 kHzand 2 MHz, and yet more preferably between 1 MHz and 2 MHz highfrequency ultrasound to create a standing wave, i.e. regions of minimalpressure (nodes) and maximal pressure (antinodes) within a liquid filledseparation chamber. Whilst not wishing to be limited to any one theory,the Inventor's consider that when using this method, suspended particlesor droplets migrate specifically towards one of these two regions due toacoustic radiation forces, based on their density and compressibility.In general, the aggregated MOFs are slightly denser than the surroundingfluid, and migrate towards the pressure nodes. This gathering of MOFmaterial enhances the tendency to form larger aggregates which thensediment at a greatly accelerated rate to the bottom of the separationchamber, where they can be collected.

Ultrasonic and/or megasonic operation also has the ability to achievespecificity of separation based on particle size by tuning of theoperation parameters such as frequency and energy density.

Ultrasonics and/or megasonic operation involves no moving parts, and canhave a low surface area of contact with the fluid providing a lowercapacity for fouling, and ease of cleaning. A separator according to thepresent invention essentially comprises a housing or container in whicha liquid reservoir can be formed. The liquid reservoir is filled withthe MOF containing solution produced by the tubular flow reactor of thepresent invention. A high frequency transducer, such as a platetransducer is either submerged in the liquid filled reservoir or engagedwith a wall of reservoir to project ultrasonic and/or megasonicfrequencies through the MOF containing solution for a certain length oftime to effects the desired separation of MOF from solution and/orseparation of contaminants from the MOF into the solution.

The Applicant considers that the size, material and/or geometry of theseparation vessel or housing (which may, in some cases be housed withinthe tubular reactor) may have an effect on the outcome (degree,efficiency or the like) of the separation process of MOF. Similarly, thepositioning, arrangement and alignment of transducers within aseparation apparatus may have an effect on the outcome (degree,efficiency or the like) of separation process of MOF.

The Applicant notes that ultrasonics and megasonics are a well knowseparation technique for particles, particularly in the biotechnologyand food processing areas. Previous applications of ultrasonics andmegasonic involved liquid/liquid and solid/liquid separation especiallyin food processing (milk fat separation and palm oil separation).However, the Inventors are not aware of any previous published workusing ultrasound, in particular megasonics, for the combined separation,washing, and/or activation of any porous material.

The inventors believe that the ultrasonic and megasonic ranges of thepresent invention provide at least one of surface area improvement,separation and/or washing properties for MOF containing solutions. Thedifference between ultrasonic and megasonics lies in the frequency thatis used to generate the acoustic waves. Ultrasonic uses lowerfrequencies (20 to 400 kHz) and produces random cavitations. Megasonicuses higher frequencies frequency (>0.4 MHz to several MHz) and producescontrolled and smaller cavitations which allows the separation ofnanocrystals (in our case, the MOFs). Furthermore, higher megasonicfrequencies do not cause the violent cavitation effects found withultrasonic frequencies. This significantly reduces or eliminatescavitation erosion and the likelihood of surface damage to the productbeing cleaned.

EXAMPLES

The production of five studied MOF, copper trimesate (HKUST-1),zirconium terephthalate (UiO-66), scandium biphenyl-tetracarboxylate(NOTT-400), aluminium fumarate (Al-fum) and aluminium terephthalate(MIL-53) using process, system and apparatus according the presentinvention, will now be exemplified by example. However, it should beappreciated that the present invention is suitable for use with a largenumber of MOFs and should therefore not be limited to the exemplifiedMOF structures in these example. The examples provided can therefore bemore generally applied to a wide range of MOFs.

Example 1—Synthesis of HKUST-1, UiO-66 and NOTT-400

To demonstrate the effectiveness and the versatility of this approach,three different families of MOFs have been synthesized: copper trimesate(HKUST-1), zirconium terephthalate (UiO-66) and scandiumbiphenyl-tetracarboxylate (NOTT-400). These three MOFs are thermally andchemically stable crystals which represent some of the most interestingmaterials for potential applications in gas storage and catalysis.

A schematic of the overall experimental apparatus is shown in FIG. 2A.The apparatus 300 and production method uses a commercially availableflow chemistry synthesis platform (Vapourtec® R2+/R4 see below) tosimultaneous pump separate precursor solutions of the organic ligand 301and the metallic salt 302 into a T-micro mixer 304 via HPLC pumps 303.The mixed solvent streams were combined and directed into reactor 305which comprised coiled flow reactors consisting of one to four (in thiscase one) 1.0 mm ID perfluoroalkoxy polymer (PFA) coil modules connectedin series.

Experiments were performed using a commercially available continuousflow reactor Vapourtec R2+/R4 (www.vapourtec.co.uk) consisting of twoPFA polymer tubular reactors used in a typical mesoscale synthesis ofMOFs. The system comprises the pumping and reagent selection module (topstage) and the four channel air-circulated heating reactor coils (lowerstage). In a typical synthesis of metal-organic frameworks, separatesolutions of the precursors are directed into the reactor by HPLC pumps303 through a T-type static mixer 304 to promote complete mixing of theseparate reagent streams. The combined mixed reactants are then directedinto the heated reactor zone of the Vapourtec R4 unit which comprisescoiled tubular reactors 305 fabricated from perfluoroalkoxy polymer(PFA) tubing (internal diameter of 1 mm and a volume of 10 mL for eachtubular reactor 305). Where required, the reactor volume can be readilyincreased by connecting the coiled reactor tubes in series (up to fourcoils for a single Vapourtec R4 unit). On exiting the reactor zone, thestream is passed through a back-pressure regulator 307 (Upchurch) (100psi) to maintain constant the pressure of the flow stream. The exitingproduct stream was then collected into a volumetric flask 306 (100 mL)whereupon it was cooled to room temperature.

Each reactor coil 305 has a volume of 10 mL and its temperature isregulated to be constant and homogenous throughout the reaction,eliminating the possible temperature gradients often observed in batchreactors.

As noted below, the synthesis of HKUST-1 was performed in a total volumeof 20 mL at 80° C. and at total flow rates of 2, 10 and 20 mL·min⁻¹,which resulted in a residence time of 10, 5 and 1 min respectively.UiO-66 was also successfully synthesized using the same set-up but at130° C. in 10 min and using a flow rate of 2 mL·min⁻¹ and NOTT-400 at85° C. in 15 min, at 2 mL·min⁻¹, using a total volume of 30 mL.

Synthesis of HKUST-1 Using Vapourtec R4/R21 Reactor

In a typical reaction, solutions of 0.1 M Cu(NO₃)₂.3H₂O and 0.24 Mbenzene-1,3,5-tricarboxylic acid (BTC) both in ethanol, were pumped intothe flow reactor (PFA tubing, 20 mL). The synthesis was conducted at 80°C. using three total flow rates of 2, 10 and 20 mL·min⁻¹ giving aresidence time of 10, 5 and 1 min respectively and at 140° C. using aflow rate of 20 mL·min⁻¹. The material was washed twice with ethanol anddried under vacuum for 8 hours at 40° C. Yield: 74% for 2 mL·min⁻¹ at80° C.; 61% for 10 mL·min⁻¹ at 80° C.; 58% for 20 mL·min⁻¹ at 80° C.;89% for 20 mL·min⁻¹ at 140° C.

Synthesis of UiO-66 Using Vapourtec R4/R21 Reactor

In a typical reaction, the two reactants were 0.1 M ZrCl₄ and 0.1 M1,4-tricarboxylic acid (BDC), both of them prepared in dimethylformamide(DMF).

The total volume was 20 mL. The synthesis was conducted at 130° C. andwith at combined flow rate of 2 mL·min⁻¹ yielding a residence time of 10min. The material was washed once with DMF and immersed in methanol bathfor 2 days. The final product was dried under vacuum for 8 hours at 40°C. The resulting yield was 67%.

Synthesis of NOTT-400 Using Vapourtec R4/R21 Reactor

In a typical reaction, 0.04 MSc(SO₃CF₃)₃ and a 0.08 MBiphenyl-3,39,5,59-tetracarboxylic acid (H4BPTC) were prepared in amixture of DMF, tetrahydrofuran (THF) and water and were pumpedcontinuously into the flow reactor. The total reactor volume was 30 mL.The synthesis was conducted at 85° C. and with an individual flow rateof 1 mL·min⁻¹ giving a residence time of 15 min. The material was washedonce with DMF and immersed in acetone bath for 1 day. The final productwas dried under vacuum for 8 hours at 40° C. The resulting yield was61%.

Characterisation

Scanning electron microscopy (SEM) images were collected on a Quanta 400FEG ESEM (FEI) at acceleration voltage of 0.2-30 kV. Copper was used assupport. The X-ray powder diffraction (XRPD) measurements were performedwith an X'Pert Pro MPD diffractometer (Panalytical) over a 2θ range of5° to 45°. The thermogravimetric analysis (TGA) was performed on aPerkin-Elmer STA-600 under a constant flow of N₂ at a temperatureincrease rate of 5° C./min. Gas adsorption isotherms for pressures inthe range 0-120 kPa were measured by a volumetric approach using aMicrometrics ASAP 2420 instrument. All the samples were transferred topre-dried and weighed analysis tubes and sealed with Transcal stoppers.HKUST-1, UiO-66 and NOTT-400 were evacuated and activated under dynamicvacuum at 1026 Torr at 140° C. for 8 hours, 120° C. for 12 hours and170° C. for 12 hours respectively. Ultra-high purity N₂ and H₂ gaseswere used for the experiments. N₂ and H₂ adsorption and desorptionmeasurements were conducted at 77 K. Surface area measurements wereperformed on N₂ isotherms at 77 K using the Brunauer-Emmer-Teller (BET)model with adsorption values increasing range of 0.005 to 0.2 relativepressures. In order to estimate the particle size of the MOFs astatistical study was done based on five different SEM images of eachMOFs.

Results

Conventional batch synthesis requires between 24 h for the production ofHKUST-1 and UiO-66 and 72 h for NOTT-400. The reaction times using thecontinuos flow reactors are therefore an improvement over the batchsynthesis results. These short reaction times are made possible by thehigh surface-area-to-volume ratio in the reactor which is much higherthan that of a typical bottom flask used in solvothermal synthesis. Thedimensions of the flow reactor (1 mm ID) ensure an excellent heat andmass transfer showing a narrow residence time distribution and a nearplug-flow like profile.

The hourly rate of MOF production of the synthesis was calculated toevaluate the impact of the continuos flow approach on larger scaleproduction. The results are provided in Table 1, which provides theresults of the present invention compared to other candidates for largerscale production of MOFs and commercially produced HKUST-1 sourced fromthe listed literature sources.

TABLE 1 Comparisons of the reaction time between MOFs synthesized byconvectional batch and by flow chemistry. BET surface areas, grams ofMOF produced per 1 hour using flow chemistry and space time yield (STY).Reaction SABET STY MOF time (m²g⁻¹) g · h⁻¹ (kg m⁻³d⁻¹)^(f) HKUST-1^(a)1 min 1852 1.48 592 HKUST-1^(b) 5 min 1673 2.04 n/a Basolite C300^(b)150 min 1820 n/a 225 UiO-66^(a) 10 min 1186 1.68 672 UiO-66^(d) 24 h1147 n/a n/a NOTT-400^(a) 15 min 1078 2.78 741 NOTT-400^(e) 72 h 1350n/a n/a ^(a)Vapourtec Flow chemistry reactor (Mesoscale). ^(b)Data fromref. Faustini, M. et al. Microfluidic Approach toward Continuous andUltra-Fast Synthesis of Metal-Organic Framework Crystals andHetero-Structures in Confined Microdroplets. J. Am. Chem. Soc.135,14619-14626 (2013). c- Data from ref. Mueller, U. et al.Metal-organic frameworks-prospective industrial applications. J. Mater.Chem. 16, 626-636 (2006). ^(d)Data from ref. Cavka, J. H. et al. A NewZirconium Inorganic Building Brick Forming Metal Organic Frameworks withExceptional Stability. J. Am. Chem. Soc. 130,13850-13851 (2008),^(e)Data from ref. Ibarra, I. A. et al. Highly porous and robustscandium-based metal-organic frameworks for hydrogen storage. Chem.Commun. 47, 8304 (2011). ^(f)Space-time yields given in this table basedon the volume of the reaction mixture in 8 hours.

The reaction rate values obtained by a flow chemistry approach of thepresent invention are many multiples higher than any other valuesreported in the literature. This fact underlines the great potential ofcontinuous flow processing for industrial production of MOF materials,especially bearing in mind that the setup allows to continuously producematerial for extended periods of time without observable blocking of thereactor coil or back-pressure regulator.

The overall quality of the produced HKUST-1, UiO-66 and NOTT-400crystals was confirmed using X-Ray powder diffraction (XRPD). Thediffraction patterns shown in FIG. 3 confirm that the purity of thecrystals obtained by flow chemistry is identical to the crystalssynthesized by conventional solvothermal methods. The thermogravimetricanalysis (TGA) curves show a continuous weight loss over thetemperatures ranges 50 to 100° C. due to the solvent loss, with smalldifferences due to the type of solvent used in the purificationprocesses. The size and morphology of the crystals were corroborated byscanning electron microscopy (SEM), as shown in FIGS. 3 and 6.

The typical octahedral HKUST-1 crystals are obtained using differentresidence times and temperatures, where lower flow rates yielded moreideal crystal shapes (see FIG. 4). For UiO-66, small crystals under 100nm are obtained, while for NOTT-400 rectangular crystals below 10 mm areobtained. These crystal sizes, as with other faster syntheticmethodologies like microwaves, are smaller than the crystals obtainedunder standard solvothermal conditions. This effect is attributed to therapid crystallization kinetics induced by the flow chemistry approach.Standard N₂ and H₂ adsorption measurements proved the porous characterof the MOFs and yielded BET (Brunauer, Emmett and Teller) surface areasthat are similar to values obtained by conventional methods, somemesoporosity was witnessed in UiO-66 due to inter-particle packingbetween the nano-sized crystallites.

The continuous flow chemistry set-up employed in the present case isamenable to precise control over reaction parameters. Taking advantageof this, a detailed investigation of different reaction conditions wasundertaken to elucidate to what point facile and commercially attractiveconditions (i.e. low temperatures, high concentrations, short residencetimes) could be employed prior to a loss of production quality, which inFIG. 4 is defined as the result of yield multiplied by surface area,normalised to a value between zero and one. Control of particle size isalso attractive for tailoring MOF production to a specific application,without the need for bespoke equipment. For example, use in mixed matrixmembranes requires nanoparticulate materials, whereas bulk applicationssuch as gas storage are better suited to macroscale particles that arenot flocculent.

The results indicate (FIGS. 4, 5 and 6) that reaction temperature is thekey factor affecting product quality, with both yield and surface areacorrelated in this case. Higher copper concentrations moderate theyield, but surface areas were largely unaffected. Encouragingly,reducing residence time appeared to improve surface areas withoutdiminishing yields. In this case, the increase in surface area could beaccounted for by a corresponding decrease in particle size (FIG. 4,top). This type of control over particle size distribution from 100 nmto 100 μm is of paramount importance for many applications, such asadsorption and catalysis.

The continuous reaction apparatus of the present invention therefore ledto the rapid production of three separate MOFs, namely HKUST-1, UiO-66and NOTT-400. This can be achieved without loss in product quality, withprocess optimisation leading to unprecedented production efficiency asmeasured by space-time yields, and control over particle size without aloss of surface area or yield.

Example 4—MOF Synthesis and Megasonic Separation

Aluminium fumarate (Al-fum) and aluminium terephthalate (MIL-53) weresynthesized using flow chemistry technology following the methodologyoutlined in Example 1.

A schematic representation showing the general flow reactor setup usedin this example is shown and described above in relation to FIG. 2A. Thereactor 405 used in this setup is shown in FIG. 2B, and has beendescribed in detail above.

Here after mixing in T-mixer 304, the organic ligand and metal ions insolution with a solvent, preferably water and/or mixture of water andethanol, at temperature from about 25° C. to about 130° C. (depending ofthe MOF synthesis) are then directed into a heated tubular flow reactor305 (FIG. 2A) and 405 (FIG. 2B). The specific coil flow reactor 405(FIG. 2B) used in this application had a 108 mL capacity with 6.0 mm IDstainless steel tube with a total flow rate of 90 mL min⁻¹. A MOF streamis obtained from the flow reactor 405/305 and is cooled to roomtemperature using a water bath heat exchanger.

It is noted that higher ligand concentration increase yields, however,increase also the risk of blockage in the flow reactor 405/305.

A MOF stream is obtained and is cooled to room temperature using a waterbath heat exchanger (not illustrated). If desired, the solvent can bereused by recycling after the first separation stage. This isparticularly attractive for recycling the unreacted ligand which isusually the most expensive reactant, or when an expensive or toxicsolvent is used.

Wash and separation stages (again not illustrated) are performedpreferably with water and/or with mixture of water and ethanol. Aportion of the washing medium can be recycled back to the reactor305/405, while the remaining liquid is sent to waste. Depending on thereaction conditions the recycle and waste streams consist of solvent,unreacted ligand and salt, as well as a reaction byproduct. Theby-product concentration depends on the recycle flow rate. Note thathigh concentrations may have a detrimental effect on the MOF synthesisreducing the yield, dictating the maximum feasible recycle flow rate.

The MOF crystals formed in Example 4 were isolated from the solventusing a megasonic apparatus and process according to one embodiment ofthe present invention. A conventional centrifuge was used as a controlreference.

The megasonic separator 500 is shown in FIG. 7. The megasonic separator500 applies >400 kHz high frequency ultrasound to create a standingwave, i.e. regions of minimal pressure (nodes) and maximal pressure(antinodes) within a separation chamber 510 of megasonic separator 500.

FIG. 7(a) shows the megasonic separator 500 set-up with a high frequencysystem using one 200 kHz plate transducer 505 (best shown in FIG. 7(b)).The megasonic separator 500 essentially comprises a 1.1 L stainlesssteel container. It should be noted that a clear polycarbonate 6-litrecontainer shown in the Figures was used initially to visualize theseparation process. However, normal operation and experiments wereperformed in a 1.1-litre stainless steel container (not pictured).

The illustrated clear polycarbonate 6-litre container is split into twosections, a 1.1 L treatment section 510 containing the transducer plate505 and an unprocessed section 512. The treatment section 510 andunprocessed section 512 are separated by a metallic (stainless steel)reflector plate 515 used to reflect the megasonic waves.

The plate transducer 505 was used for sonication at a frequency of 2 MHz(305 W) for 10 min.

FIG. 7c shows the schematic of the standing wave pattern formed by thesuperimposition of a reflected sound wave within the treatment section510. The separation distance between adjacent nodes or antinodes, ishalf a wavelength. Depending on the specific density and compressibilityof the particles, they will collect either in the nodal (top, blackdotted planes) as for the bright yellow particles or antinodal (bottom,red dotted planes) pressure planes as for the darker yellow particles.As previously noted, suspended particles or droplets migratespecifically towards one of these two regions due to acoustic radiationforces, based on their density and compressibility. In general, theaggregated MOFs are slightly denser than the surrounding fluid, andmigrate towards the pressure nodes. As shown in FIG. 8, this gatheringof MOF material enhances the tendency to form larger aggregates whichthen sediment settles at a greatly accelerated rate to the bottom of theseparation chamber, where they can be collected.

FIG. 8 provides three photographs of a MOF solution being treated in amegasonic treatment apparatus 500 shown in FIG. 8(a) at specific times(1 minute, 4 minute and 10 minutes) during the megasonic separationprocess described above. In the left or separation compartment 510, themegasonic separation and purification process of the Al-MOF is shown.The right compartment 512 shows the same MOF solution withoutsonication. The settling of the MOF is clearly visible in the separationcompartment 510 after 4 mins and 10 mins compared to the cloudiness ofthe same MOF solution without sonication shown in the right compartment512.

Example 5—Investigation into Changes in MOF Composition

In order to investigate whether megasonics separation introduces changesin the MOF composition, ζ-potential measurements were recorded aftereach washing step of Example 5 as shown Table 2.

TABLE 2 ζ- Potential of the Al-Fumarate and MIL-53 MOF material aftereach wash step using Megasonics using water as a dispersant. MOF washingprocess (Megasonics) ζ- potential (mV) Al-Fumarate flow reactor  +8.3 ±0.4 Al-Fumarate wash 1 in H₂O  +8.8 ± 0.0 Al-Fumarate wash 2 in H₂O +8.8 ± 0.1 Al-Fumarate wash 3 in H₂O  +8.9 ± 0.2 Al-Fumarate wash 4 inEtOH +10.6 ± 0.2 Al-Fumarate wash 5 in EtOH +11.3 ± 0.8 MIL-53 flowreactor +13.3 ± 0.4 MIL-53 wash 1 in H₂O +15.1 ± 0.5 MIL-53 wash 2 inH₂O +14.7 ± 0.3 MIL-53 wash 3 in H₂O +12.6 ± 0.5 MIL-53 wash 4 in EtOH+12.7 ± 0.2 MIL-53 wash 5 in EtOH +14.6 ± 0.1

No significant changes to the surface charge were observed, pointing toa separation that is based on reversible aggregation.

To determine the quality of the crystals, XRPD and SEM measurements ofthe MOFs separated with megasonics and by the standard lab-scalecentrifuge were compared. X-Ray powder diffraction (XRPD) confirmed thecrystallinity of our Al-fum and MIL-53, showing identical patterns tothose of crystals synthesized by solvothermal methods. It was observedby scanning electron microscope that the high-frequency treatment alsodoes not affect the size and shape distribution of the MOFs.

A comparison of the backscattering and transmission data of thesupernatant collected from the first separation of the MOF solutionusing centrifuge and megasonics was undertaken as shown in FIG. 9. Asshown in FIG. 9, the recoverable MOF yield obtained with megasonicseparation compared to the conventional centrifuge method is 3% less foreach washing step. This difference can be attributed to the fact thatcentrifuge separation generates a higher G-force compared to thesettling by gravity in megasonics, which leads to a more effectiveremoval of the MOF material.

The measurements of the BET surface areas revealed that the MOFsseparated and washed with megasonics showed a drastic increase of 21%for the Al-Fum and 47% for MIL-53 over standard centrifuge washed MOF,which had BET values similar to literature (see Table 3).

TABLE 3 Comparisons of the reaction time between MOFs synthesized byconvectional batch (using water as a reaction solvent) and by flowchemistry. BET surface areas, grams of MOF produced per 1 hour usingflow chemistry and STY. Full adsorption isotherms are provided in thesupplement information. STY Reaction Yield (kg · SA_(BET) time g h⁻¹ (%)m³ · d⁻¹) (m² g⁻¹) From reactor Al-fum 1.2 min 338.04 109.0 25,040 —MIL-53 1.2 min 50.68 112.8 3,754 — Centrifuge × 5 Al-fum 1.2 min 281.8890.9 20,880 890 MIL-53 1.2 min 42.14 93.8 3,121 806 Megasonics × 5Al-fum 1.2 min 225.07 72.6 16,672 1075 MIL-53 1.2 min 35.10 78.1 2,6001183 Commercial^(a) Al-fum 10.2 min 174 86 5339 1140 Literature^(b)MIL-53 4 hours 125 86 1300 1010 ^(a)M. Gaab, N. Trukhan, S. Maurer, R.Gummaraju and U. Müller, Microporous Mesoporous Mater., 2012, 157,131-136. ^(b)P. A. Bayliss, I. A. Ibarra, E. Pérez, S. Yang, C. C. Tang,M. Poliakoff and M. Schröder, Green Chem., 2014, 16, 3796.

The Inventors attribute this improvement to the enhanced mass transferthat arises from acoustic streaming during megasonic application thatpromotes the removal of the excess organic ligands molecules inside ofthe pores. This is an important step forward for cost-effective andgreen production of MOFs as similar surface areas have only beenobtained using laboratory scale methods that would be expensive at largescale, namely by using supercritical ethanol or calcination up to 330°C.

The preceding Examples indicates that the apparatus, process and systemof the present invention provides the following advantages:

-   -   Reaction time: Flow chemistry is able to produce MOFs at        dramatically reduced reaction times, e.g. HKUST-1 in 1 min as        opposed to 24 h using traditional methods, or NOTT-400 in 10 min        rather than 72 h;    -   Space Time Yield: The Space Time Yield obtained by the process        and apparatus of the present invention is 10 times larger than        commercial employed methods; and    -   Green chemistry principles: The present invention (reactor and        megasonic separation) follow green chemistry principles leading        to improved workplace safety and lower environmental impact.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is understood that the invention includes allsuch variations and modifications which fall within the spirit and scopeof the present invention.

Where the terms “comprise”, “comprises”, “comprised” or “comprising” areused in this specification (including the claims) they are to beinterpreted as specifying the presence of the stated features, integers,steps or components, but not precluding the presence of one or moreother feature, integer, step, component or group thereof.

The invention claimed is:
 1. A process for producing metal organicframeworks, the process comprising: mixing at least two differentprecursor solutions for forming the metal organic framework (MOF)through inline mixing to form a solution mixture, the precursorsolutions comprising a first precursor solution comprising at least onemultidentate linking ligand in solvent; and a second precursor solutioncomprising a metal cluster or metallic salt thereof in solvent,introducing the solution mixture into an apparatus which comprises: atubular flow reactor which comprises a tubular body having an inlet intowhich, in use, the solution mixture is fed and flows, said tubular bodyincluding at least one annular loop comprising a coil; and a flowrestriction device comprising a back-pressure controller downstream ofthe tubular reactor for controlling the pressure within the tubularreactor, and promoting a reaction within the tubular flow reactor toform the metal organic framework, wherein the precursor solutions aremixed through inline mixing in a feed conduit fluidly connected to theinlet of the tubular body and the resulting solution mixture being fedinto said inlet at room temperature, the two or more precursor solutionsbeing mixed at or proximate said inlet, and wherein the solution mixtureflows through said tubular body mixing the precursor compounds thereinto produce the metal organic frameworks.
 2. The process according toclaim 1, wherein the apparatus further comprises an inline mixer locatedat or proximate the inlet to the tubular body, the inline mixer mixingthe precursor solutions through inline mixing in said feed conduitfluidly connected to the inlet of the tubular body.
 3. The processaccording to claim 1, further including the step of: applying a highfrequency ultrasound of at least 20 kHz to a MOF containing solutionsourced from the tubular flow reactor, thereby separating the MOFmaterial from solution as an aggregated sediment which settles out ofthe MOF containing solution.
 4. The process according to claim 3,wherein the yield of MOF from the apparatus is greater than 60%.
 5. Theprocess according to claim 1, wherein the tubular body comprises aheated tubular body in which the solution mixture is heated during flowthrough the said tubular body, said flow through said tubular bodymixing the precursor compounds therein to produce the metal organicframeworks.
 6. The process according to claim 5, wherein the tubularbody is located inside a heated housing and the housing is heated viaheating inlet and outlet port connections through which heated fluid isfed and extracted to heat the tubular body.
 7. The process according toclaim 5, wherein the tubular body heats the precursor compounds to atemperature of between 20 and 200° C.
 8. The process according to claim1, wherein the apparatus further includes a separator for separating ametal organic framework (MOF) from a solution, comprising: a housinghaving a reservoir capable of receiving a MOF containing solution; and ahigh frequency ultrasound transducer operatively connected to thereservoir and capable of applying megasonic frequencies of at least 400kHz to the MOF containing solution.
 9. The process according to claim 8,wherein the housing comprises a container including at least one wallposition to contact the MOF containing, and the transducer is highfrequency ultrasound transducer is position within the reservoir or inengagement with the at least one wall.
 10. The process according toclaim 8, wherein the housing includes at least one reflector surfacedesigned to reflect the applied megasonic frequencies within thereservoir.
 11. The process according to claim 8, wherein the appliedhigh frequency ultrasound is at least one of: between 400 kHz to 4 MHzor greater than 1 MHz.
 12. The process according to claim 8, wherein theapplied high frequency ultrasound is at least one of: between 600 kHzand 2 MHz; or between 1 MHz and 4 MHz.
 13. The process according toclaim 8, wherein at least one contaminant includes occluded unreactedligands within pores of the MOF.
 14. The process according to claim 1,wherein the metal organic framework comprises a plurality of metalclusters, each metal cluster including one or more metal ions; and aplurality of charged multidentate linking ligands connecting adjacentmetal clusters, and wherein the at least two different precursorsolutions comprise a first precursor solution comprising at least one ofthe multidentate linking ligands; and a second precursor solutioncomprising a metal cluster or a metallic salt thereof.
 15. The processaccording to claim 14, wherein each metal cluster comprises two or moremetal ions and each ligand of the plurality of multidentate ligandincludes two or more carboxylates.
 16. The process according to claim14, wherein at least one ligand of the plurality of multidentate ligandcomprises an organic ligand which is at least bidentate and is selectedfrom the group consisting of formic acid, acetic acid, oxalic acid,propanoic acid, butanedioic acid, (E)-butenedioic acid,benzene-1,4-dicarboxylic acid, benzene-1,3-dicarboxylic acid,benzene-1,3,5-tricarboxylic acid, 2-amino-1,4-benzenedicarboxylic acid,2-bromo-1,4-benzenedicarboxylic acid, biphenyl-4,4′-dicarboxylic acid,biphenyl-3,3′,5,5′-tetracarboxylic acid, biphenyl-3,4′,5-tricarboxylicacid, 2,5-dihydroxy-1,4-benzenedicarboxylic acid,1,3,5-tris(4-carboxyphenyl)benzene, (2E,4E)-hexa-2,4-dienedioic acid,1,4-naphthalenedicarboxylic acid, pyrene-2,7-dicarboxylic acid,4,5,9,10-tetrahydropyrene-2,7-dicarboxylic acid, aspartic acid, glutamicacid, adenine, 4,4′-bypiridine, pyrimidine, pyrazine,pyridine-4-carboxylic acid, pyridine-3-carboxylic acid, imidazole,1H-benzimidazole, 2-methyl-1H-imidazole, and mixtures thereof.
 17. Theprocess according to claim 14, wherein the metal ion is selected fromthe group consisting of Group 1 through 16 metals of the IUPAC PeriodicTable of the Elements including actinides, and lanthanides, andcombinations thereof.
 18. The process according to claim 14, wherein themultidentate linking ligand has 6 or more atoms that are incorporated inaromatic rings or non-aromatic rings.